The present invention relates to insulation materials and, more particularly, to laminated molded insulation material for slot insulation in dynamoelectric machines.
Although the present invention is applicable to dynamoelectric machines functioning as either motors or generators, for concreteness of description, the following disclosure is directed to a slot armor for an electric generator. It will be understood that the problems to be solved and the embodiments of the invention disclosed herein to solve such problems are equally applicable to appropriate motors.
Industrial and utility electric generators are conventionally constructed with a DC field winding disposed in the rotor and AC windings disposed in the stator surrounding the rotor. The rotor is typically a large, one-piece metal forging having axial slots machined in its surface. Copper conductors are longitudinally disposed in the axial slots for providing DC excitation to the rotor. The copper conductors extend axially beyond the slots in order to permit the formation of end turns for interconnecting the copper conductors into a closed circuit and for providing connection to slip rings which supply the excitation power.
An insulating slot armor is conventionally disposed between the copper conductors and the sides of the slot in order to prevent grounding the excitation voltage to the rotor forging and also to withstand electric fields present in the rotor. In the electric generators of interest, the slot armor is a rigid molded insulating material which may take the form of two L-shaped molded structures disposed in the slot with the short legs of the L shapes facing each other in the bottom of the slot.
One slot armor of the prior art is disclosed in U.S. Pat. No. 3,974,314 which employs a high-dielectric film protected on each side by a layer of an unwoven aramid paper such as, for example, an aramid paper sold under the name Nomex. Further protective layers of a glass-fiber/epoxy laminate are disposed on the outside of each layer of aramid paper to form, at the least, a five-layer sandwich with the high-dielectric film in the center between, first the two aramid-paper layers, and then the two glass fiber/epoxy layers. Conventionally, each glass fiber/epoxy layer may be laid up as a single layer or multiple plies of woven or non-woven fabric or combinations of woven and non-woven fabric as required.
A corresponding slot armor is disclosed in U.S. Pat. No. 4,162,340 which adds thickening plies of glass fiber/epoxy at desired locations. The fundamental sequence of materials in this material is the same as in the previously referenced patent, that is, a central high-dielectric film protected by aramid paper and glass fiber/epoxy layers.
The amount of copper required to conduct the DC excitation power in a typical generator rotor is quite massive. When such large mass is combined with a high rotational speed of, for example, 3600 RPM, the copper experiences a large radial acceleration tending to force it radially outward. To resist the outward forces developed by the radial acceleration, the portions of the copper conductors that lie within the axial slots are wedged tightly in position using machined wedges fitting into dovetail slots machined into the radially outer extremities of the slots.
The portions of the copper conductors extending axially beyond the axial slots of the rotor forging to form the end turns are held in place against radial forces by retaining rings at each end of the rotor. One conventional generator employs a disk-shaped centering ring shrink-fitted onto the generator shaft axially outward from the end turns and a cylindrical retaining ring shrink-fitted at its axially outer extremity onto the centering ring. When a rotor of this construction is started, the radially outward forces applied to the retaining ring by the copper conductors are sufficient to expand the axially inner end of the retaining ring as much as a few thousandths of an inch. This expansion permits the copper conductors to also move outward slightly. The motion of the copper conductors may include portions just inside the axial slots as well as portions just outside the axial slots.
A base-load electric generator is one which, once it is started and placed on line, often remains in constant operation without stopping for periods measured in years. A peaking generator, on the contrary, is started up from a stop whenever its output power is needed to augment the power of the base load system. When its power output is no longer required, a peaking generator may be stopped. An industrial generator may be similarly operated.
We have observed that, after a few hundred to a few thousand start-stop cycles of a peaking or industrial generator, the glass fiber outer layers of the slot armor of the prior art become abraded just inside and just outside the axial slots by the friction between themselves and the copper conductors or between themselves and the sides of the slots. As the slot armor is abraded away, its insulating porperties become degraded. Such degradation of the insulating properties can eventually lead to electrical breakdown through the slot armor. We believe that the observed abrasion of the slot armor occurs due to the above-described radially outward motion of the copper conductors during generator startup and the corresponding radially inward motion during stopping.
The layers of aramid paper included in prior art slot armors contributes some undesirable properties to the slot armor. The aramid paper layer is sometimes not well bonded in the thickness dimension and therefore permits interlaminar bubbles and separation to form. In addition, the aramid paper layer provides substantial thermal insulation which resists the discharge of heat from the copper conductors through the slot armor to the rotor forging.